1. Field of the Invention
The present invention relates to a rotation sensor, and more particularly, to a rotation sensor which is excellent in detection accuracy and easy to manufacture.
2. Description of the Related Art
A rotation sensor is known, which serves to detect a relative rotation angle of two shafts arranged for relative rotation. The rotation sensor is designed, for example, in the form of a torque sensor for detecting rotation torque applied to an input shaft (steering shaft) of an automotive power steering apparatus, more specifically, for detecting rotation torque applied to a torsion bar of the input shaft through which first and second shafts of the input shaft are coupled for relative rotation. The rotation sensor is utilized for electronic control of the power steering apparatus (see, Japanese patent publication no. 7-21433, for instance). Meanwhile, the first shaft is coupled to a steering wheel, whereas the second shaft is coupled through the power steering apparatus to wheels, for instance.
The rotation sensor is comprised of a first rotor coupled to one of the first and second shafts, a second rotor coupled to the other of these shafts, and a stationary core accommodating therein exciting coils. The first rotor is constituted by magnetic material members and electrically conductive layers that are alternatively disposed at equal intervals in the circumferential direction. The second rotor is constituted by a metal member formed with notches that are equally spaced from one another in the circumferential direction. Depending on the relative rotational position of the first and second rotors, an area, for which the conductive layers of the first rotor traverses a magnetic field formed by the exciting coils supplied with AC current, changes, to thereby change eddy currents generated in the conductive layers. Based on the resultant change in effective inductances of the exciting coils, the rotation sensor detects a relative rotation angle of the first and second shafts or rotation torque applied to the torsion bar.
In order to suppress affections of disturbances such as a temperature variation on the detection accuracy, the rotation sensor obtains, as a detection output, a difference between outputs of two exciting coils. Preferably, these two exciting coils are the same in construction and spaced apart from each other with an adequate spacing in the direction of the rotation axis so as not to affect with each other. The first and second rotors have their lengths, measured in the direction of the rotation axis, that are about several millimeters greater than that of the exciting coils, to avoid influences caused when the rotation sensor vibrates in the direction of the rotation axis.
The aforementioned conventional rotation sensor entails various problems as described below.
First, to satisfy requirements that an automobile be light in weight and compact in size, a small-sized rotation sensor is requested. To this end, an attempt is made to shorten a spacing between the two exciting coils in the direction of the rotation axis. In such an arrangement, however, when vertical vibration of the rotation sensor is caused during traveling of an automobile equipped with the rotation sensor, relative vertical motions occur between the stationary core receiving the two exciting coils and the first and second rotors. As a result, one of the exciting coils has a positional relationship with respect to the first and second rotors that is different from a relationship between the other exciting coil and the rotors, and hence the inductances of the two exciting coils change in opposite directions. Consequently, the detection output of the rotation sensor, which is the difference between outputs of the two exciting coils, becomes excessively greater than actual rotation torque, so that a large detection error may be caused.
With reference to FIG. 1, explanations will be given as to detection errors that may be caused by vibration applied to a rotation sensor having two exciting coils that are disposed close to each other.
A rotation sensor 501 shown in FIG. 1 comprises a first rotor 502, a second rotor 503, and a stationary core 504 having cores 504a, 504b of a magnetic material that accommodate therein exciting coils 504c, 504d, respectively. The first rotor 502 comprises a cylindrical member 502a of a magnetic material and a plurality of copper foils 502b. These copper foils are provided on an outer peripheral face of the cylindrical member 502a in two levels in the vertical direction so as to correspond to the two exciting coils. The copper foils located on each of upper and lower levels are spaced from one another at predetermined intervals, e.g., at intervals of a central angle of 30 degrees. The upper copper foils and the lower copper foils are disposed so as not to overlap one another in the circumferential direction. The second rotor 503 is formed with non-magnetic metal teeth 503a, corresponding to the copper foils 502b, so as to be circumferentially apart from one another.
In a case where a spacing between the two exciting coils 504c and 504d is made small to arrange the cores 504a, 504b close to each other, a slight downward movement of the first rotor 502, caused by vibration, increases an amount of shielding the magnetic flux on the side of the upper coil 504c and hence decreases the inductance of the upper coil 504c, while decreasing an amount of shielding the magnetic flux on the side of the lower coil 504d to increase the inductance of the lower coil 504d. As a consequence, the detection output of the rotation sensor which is the difference between outputs of the coils 504c, 504d becomes larger than a proper value. More specifically, a 0.1 mm downward movement of the first rotor 502 causes the detection output to be about 100 mV (corresponding to a relative rotation angle of 0.4 degrees) greater than a proper value. This indicates that a 2.5% detection error is caused for a steering shaft comprising first and second shafts that are arranged for relative rotation within a range of ±8 degrees. To be noted, a variation in the detection output should be suppressed less than 0.5% for ±0.2 mm rotor vibration in the direction of the rotor axis.
Referring to FIG. 2 showing the rotation sensor with the illustration of the second rotor 3 omitted, when vibration is applied to the rotation sensor that includes the first rotor 502 having a shortened axial length, amounts of leakage of magnetic fluxes MF from the coils 504c, 504d to the outside change in opposite directions. This makes it difficult to cancel out affections of disturbances even when a difference between two exciting coils is used as an detection output, resulting in a detection error.
In the following, another problem in the conventional rotation sensor will be discussed.
When the first and second shafts rotate together without causing relative rotation therebetween, the detection output of the rotation sensor should take a value falling within a predetermined range corresponding to rotation torque of zero. However, in the conventional rotation sensor, the permeability observed on a plane extending between the two exciting coils in the direction perpendicular to the rotation axis is circumferentially ununiform, causing circumferential ununiformity of the magnetic field generated by the two exciting coils. As a result, the detection output of the rotation sensor may fall outside the predetermined range to produce a detection error when the first and second shafts rotate without relative rotation.
In this respect, Japanese utility model publication no. 5-22836 discloses a technical art for reducing radial ununiformity of magnetic field by forming a large number of holes in a core that receives an exciting coil: By applying the proposed technique to a rotation sensor, however, circumferential ununiformity of magnetic field cannot be eliminated. For example, as for a rotation sensor having a metal casing which covers a stationary core to prevent leakage of magnetic field to the outside, a close spacing between the metal casing and the stationary core is generally ununiform in the circumferential direction, causing circumferential ununiformity of magnetic field. That is, eddy currents flowing in a surface of the metal casing are large in magnitude at locations where a small clearance is defined between the metal casing and the stationary core, thus decreasing the effective inductance of the exciting coil, whereas, at locations where a large clearance is defined between the casing and the core, the effective inductance of the exciting coil becomes large.
In order to make a close spacing between the metal casing and the core circumferentially constant to thereby eliminate the circumferential ununiformity of magnetic field, fabrication errors of the metal casing and the core with respect to perfect circles must be reduced. This requires that the metal casing and the core be fabricated with high accuracy, greatly increasing fabrication costs and making it difficult to insert the core into the metal casing.
In the rotation sensor, two exciting coils having the same characteristic may be employed in pair for temperature compensation. However, as the ambient temperature changes, there occur changes in clearances between rotation sensor elements due to the fact that these elements are different in thermal expansion or contraction from one another. This differentiates the effective inductances of the paired coils from each other, making it difficult to achieve a proper temperature compensating function.
Next, still another problem of the rotation sensor will be described.
As a magnetic material to be used to fabricate the rotor body of the first or second rotor or the core body of the stationary core, a ferrite sintered compact having an electrically conductive property, or a plastic magnetic material obtained by mixing soft magnetic powder to a thermoplastic synthetic resin, especially the plastic magnetic material, is employed since the ferrite sintered compact and the plastic magnetic material are easy to mold into a complicated shape at low costs in a short time for mass production.
For the mass production of rotors constituted by such a plastic magnetic material, dies having a mold releasing allowance (mold releasing taper) are employed, and hence the outer shape of the resultant rotor lacks the symmetry about a plane extending perpendicularly to the rotation axis and passing through the center of the rotor. In the rotation sensor having such a rotor, the magnitude of a gap defined between the rotor and the exciting coil changes depending on its position in the direction of the rotation axis, differentiating effective inductances of two exciting coils from each other. As a result, affections of disturbances cannot be sufficiently eliminated even in the rotation sensor that is designed to obtain the detection output from the difference between outputs of two exciting coils. The disturbances include a variation in ambient temperature, electromagnetic noise, a variation of oscillating frequency in an oscillating circuit for supplying an AC current to exciting coils, power source voltage, assemblage errors.
Specifically, as shown in FIG. 3, the core body 601a of the stationary core 601 is formed with an outlet port 601c through which lead wires extending from the exciting coil 601b (see FIG. 4) are drawn out to the outside. As shown in FIG. 4, the shape of the core body 601a formed at its vertically central part with the outlet port 601c is vertically symmetric. If the core body lacks such a symmetry, the resultant magnetic circuit lacks vertical symmetry, causing the detection sensitivity (inductance) of the rotation sensor to greatly vary when vibration is applied to the sensor. This may result in erroneous detection.
However, the aforementioned restrictions on the formation position of the outlet port, which must be determined to attain an improved detection accuracy, can impose strict restrictions on the design and usage of a rotation sensor.
As for another problem in a rotation sensor, countermeasures for electromagnetic wave shielding must be made to shield electromagnetic wave radiated to and from the outside, so as to meet regulations on EMC (electromagnetic compatibility), more specifically, EMI (electromagnetic interference) and EMS (electromagnetic susceptibility).
To this end, an outer face of the stationary core body made of a thermoplastic synthetic resin is generally covered by a shielding member constituted by an electrically conductive material that is excellent in electromagnetic shielding. In such a rotation sensor, a rotor makes a sliding motion relative to the shielding member. With the sliding motion of the rotor relative to the shielding member, a synthetic resin such as a plastic magnetic material that constitutes the rotor is worn out to produce resin powder, preventing smooth rotation of the rotor to lower the detection accuracy.
In the following, another problem in a rotation sensor having an electromagnetic wave shielding function will be explained.
As mentioned above, a rotation sensor of this kind comprises a stationary casing for electromagnetic shield that receives a stationary core body. The casing is constituted by an electrically conductive material such as metal, e.g., aluminum, in consideration of mechanical strength and electromagnetic shielding property.
A stationary casing 701 shown by way of example in FIG. 5 is comprised of a cylindrical casing body 701d, a cover 701e, and a spacer 701f made of an electrically conductive material such as aluminum, and accommodates therein two core bodies 701a in which exciting coils 701b are received. The spacer 701f is arranged such that the two exciting coils 701b are symmetric in shape and electromagnetic property with respect to the spacer 701f, thereby permitting the rotation sensor to properly exhibit a disturbance canceling function.
The stationary casing 701 is generally mass-produced by means of die-casting. Die-casting dies for the casing body 701d have a mold-releasing allowance (mold-release taper), and hence a three-dimensional clearance C is produced between the resultant casing 701 and the core body 701a. The magnitude of the clearance varies in dependence on the assembling accuracy of the stationary core 701, assembled by attaching the cover 701e to the casing body 701d in which the two core bodies 701a and the spacer 701f are received, and varies in dependence on the strength of a force with which the cover 701e is fixed to the casing boy 701d. 
Depending on the size of the clearance C, the magnitude and direction of eddy currents vary, which are induced in an inner face of the casing body 701d that defines a space for receiving the core body 701a . This may cause ununiformity of the inductance of the exciting coil 701b in the direction of the rotation axis. To suppress such ununiformity of the coil inductance, it is effective to make the clearance C uniform in the direction of the rotation axis, thereby enhancing the symmetry of the clearance C with respect to a plane extending in the direction perpendicular to the rotation axis and passing through the spacer 701f. However, it is extremely difficult to construct the rotation sensor in that manner.
Meanwhile, in order to draw out lead wires (not shown) of the two exciting coils 701b to the outside while reducing affections of noise as small as possible, the inductance of the lead wires should be decreased. In other words, it is preferable to shorten lengths of the lead wires to a minimum.
In the following, a further problem in a rotation sensor having an electromagnetic-wave shielding function will be described.
A stationary casing 801 shown in FIG. 6 is comprised of a casing body 803, an upper cover 804, and a lower cover 805. The casing receives two core bodies 801a in which exciting coils 801b are received. The cylindrical body 803 of the stationary casing 801 is provided at its vertically central part with a partition plate 803a such that the two exciting coils 801b are disposed to be symmetric with respect to the partition plate 803a, thereby permitting the rotation sensor to properly exhibit a disturbance canceling function. The casing body 803 is formed at its upper and lower parts with recesses 803b through which lead wires 801c of the exciting coils 801b are drawn out and connected to a printed circuit board 807 disposed vertically within a side casing 806.
Since spaces defined between the lead wires 801c and electrically conductive patterns formed on the printed circuit board 807 have their inductances, these spaces are likely to be affected by noise. As the inductances of these spaces decrease, i.e., as the lengths of the lead wires are shortened, an improved S/N ratio is obtainable in the detection by means of the rotation sensor. In order to further improve the disturbance canceling function of the rotation sensor having two exciting coils, it is preferable to use the lead wires 801c of the exciting coils having the same length. Preferably, the upper space defined between the upper lead wire 801c and a corresponding pattern formed on the printed circuit board 807 has the same projected area as that of the lower space between the lower lead wire and an associated pattern.
However, the conventional stationary core shown in FIG. 6 entails such a problem that the lead wires are long in length.
As for a rotation sensor that comprises a rotor having a cylindrical rotor body made of a magnetic material such as a plastic magnet material, the rotor is generally fabricated by affixing non-magnetic electrically conductive metal foils on an outer face of the rotor body so as to be circumferentially spaced from one another. Thus, the conventional rotor may entail drawbacks that complicated fabrication processes are required and the fabrication efficiency is low since it is difficult to affix all the metal foils in positions with accuracy.
To summarize the above explanations, a rotation sensor to which the present invention pertains comprises a first rotor mounted to one of first and second shafts arranged for relative rotation, constituted by a magnetic material, and having an outer face thereof provided with one or more conductive layers (preferably a plurality of conductive layers circumferentially separated from one another); a second rotor mounted to the other of the first and second shafts and having one or more metal members, preferably a plurality of metal members, corresponding to the one or more conductive layers of the first rotor; a stationary core fixed to a stationary member and having a stationary core body; and one or more exciting coils accommodated in the stationary core body, operable when supplied with an AC current, and having inductance thereof varying with a change in a relative rotation angle of the first and second rotors, the second rotor being disposed between the first rotor and the stationary core. The rotation sensor serves to detect a relative rotation angle of the first and second shafts or rotation torque applied therebetween based on an output of the one or more exciting coils. The detection accuracy of the rotation sensor can be lowered due to the aforementioned various factors which are shown below again.
First, as for a rotation sensor designed to detect the detection output from outputs of two exciting coils, the inductances of the exciting coils change in opposite directions when vibration is applied to the rotation sensor, causing a detection error. Further, circumferential ununiformity is present in magnetic fields generated by the two exciting coils, resulting in a detection error. Such circumferential ununiformity in magnetic fields is also caused by circumferential ununiformity in a close spacing between the stationary core and a metal casing for electromagnetic wave shield. Since the rotation sensor is constituted by various elements which are different in thermal expansion and contraction, effective inductances of the two exciting coils are differentiated as the ambient temperature changes, resulting in a detection error. Different effective inductances can also be caused when a rotor is used, which is made of a plastic magnetic and manufactured with use of dies having a mold-releasing allowance since the outer shape of the resultant rotor is asymmetric with respect to a plane extending to cross the rotor. A rotation sensor provided with a stationary core made of a plastic magnet entails restrictions on the design and usage thereof since the stationary core is required to have a vertically symmetrical shape to improve the detection accuracy. As for an arrangement having a stator core and a rotor that are constituted by a synthetic resin, with the stator core covered by a metal shielding member for electromagnetic wave shield, synthetic resin powder is produced as the rotor makes a sliding motion with respect to the shielding member, hindering smooth rotation of the rotor to lower the detecting accuracy. As for another arrangement provided with a casing for electromagnetic wave shield whose body is mass-produced by using die-casting dies having a mold-releasing margin, a gap formed between the casing body and a stationary core body received therein entails ununiformity in the direction of the rotation axis, which produces ununiformity of the inductance of an exciting coil received in the stationary core body, decreasing the detection accuracy. In addition, since the detection accuracy varies depending on the length of a lead wire extending from an exciting coil and the inductance of a space defined by the lead wire and a conductive pattern formed on a printed circuit board to which the lead wire is connected, the lead wire length must be shortened. A rotor fabricated by accurately affixing metal foils on its outer periphery with a circumferential spacing entails low manufacturing efficiency.